Three-dimensional co-axial linear photonic switch

ABSTRACT

Techniques, systems, and devices are disclosed that relate to coaxial photoconductive switch modules. The coaxial photoconductive switch may include an outer conductor, an inner conductor, and a photoconductive material positioned between the inner conductor and the outer conductor. The inner conductor, the outer conductor, and the photoconductive material have a predetermined height. A bias voltage may be applied between the inner conductor and the outer conductor. When light of a predetermined wavelength and a predetermined intensity is incident on the photoconductive material, the photoconductive material may break down allowing a current to flow through the photoconductive material between the inner conductor and the outer conductor.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

TECHNICAL FIELD

This patent document relates to systems, devices, and processes tofabricate and use photoconductive switches.

BACKGROUND

A photoconductive switch is an electrical switch that is controlled byan optical input (e.g., light) to cause photo-induced conductivity ofthe switch material. For example, light applied to the switch materialcan increase its electrical conductance as a consequence of irradiationwith light. Photoconductive switches can be used for photoconductivesampling, generation of high frequency pulses (e.g., terahertz pulses),high-speed photodetectors in optical fiber communications, and inanalog-to-digital converters, among other applications. More flexiblephotoconductive switch modules that allow independent optimization ofswitch parameters are needed to provide efficient and cost-effectivemodules.

SUMMARY

The disclosed techniques, systems, and devices relate to coaxialphotoconductive switch modules. In an example embodiment, a coaxialphotoconductive device is disclosed. The photoconductive device includesa cylindrical outer conductor having an inner radius, a cylindricalinner conductor having an outer radius, and a photoconductive material.The photoconductive material is positioned between the outer radius ofthe inner conductor and the inner radius of the outer conductor. Theinner conductor and the outer conductor are configured to establish anelectric field across the photoconductive material upon application of avoltage between the inner and the outer conductors. The coaxialphotoconductive device includes a light source having a particularwavelength or range of wavelengths coupled to the photoconductivematerial to inject light into the photoconductive material. Thephotoconductive material is shaped to have a height to allow propagationof the light from the light source through a substantial entirety of theheight of the photoconductive material, thereby causing thephotoconductive material to reduce in resistance and causing a currentto flow through the photoconductive material between the inner conductorand the outer conductor.

Also disclosed is a method for forming a photoconductive switch module.The method includes forming a coaxial structure that includes acylindrically symmetric outer conductor, a cylindrically symmetric innerconductor, and a photoconductive material filling a volume between thecylindrically symmetric inner conductor and the cylindrically symmetricouter conductor. The cylindrically symmetric inner conductor, thecylindrically symmetric outer conductor, and the photoconductivematerial are configured as a photoconductive switch between thecylindrically symmetric inner conductor, the cylindrically symmetricouter conductor. The method further includes selecting a height valuefor the coaxial structure. In some example embodiments, the innerconductor and outer conductor may have a shape different fromcylindrical. For example, the inner and/or outer conductor may betriangular, square, hexagonal, or another shape.

Also disclosed is a photoconductive switch. The photoconductive switchincludes an outer electrode, an inner electrode, and a photoconductivematerial positioned between inner electrode and the outer electrode. Theinner electrode and the outer electrode are configured to allow anelectric field to be established across the photoconductive materialupon application of a voltage between the inner and the outerelectrodes. The photoconductive material is shaped to have apredetermined height to allow, upon injection of light having aparticular wavelength or range of wavelengths into the photoconductivematerial, the light to propagate through a substantial entirety of theheight of the photoconductive material, thereby causing thephotoconductive material to break down and a current to flow through thephotoconductive material between the inner electrode and the outerelectrode.

The following features may be included in any combination. Thephotoconductive material breaks down at a breakdown voltage appliedbetween the inner conductor and outer conductor, wherein the breakdownvoltage is determined by the photoconductive material, the bias voltage,and a thickness of the photoconductive material. The inner conductor andthe outer conductor are formed from a material that includes one or moreof the following metals: titanium, gold, aluminum, silver, platinum,chromium, or copper, or one or more conductive ceramics including indiumtin oxide (ITO), aluminum doped zinc oxide (AZO), or fluorine doped tinoxide (FTO). The inner cylinder may be formed into a solid or hollowcylinder of the material. The photoconductive material includes but isnot limited to one or more of: silicon, germanium, silicon carbide,diamond, gallium nitride, gallium arsenide, gallium phosphide, aluminumnitride, boron nitride, zinc oxide, gallium oxide, or cadmium telluride.The height of the photoconductive material may be determined based on anabsorption of the incident light through the photoconductive material.For example, the height may be chosen so that 90%, or anotherpercentage, of the incident light is absorbed as the incident lightpasses through the photoconductive material. The outer conductor, theinner conductor, and the photoconductive material are each shaped tohave the same predetermined height. The photoconductive device isconfigured to allow light from the light source travel through thephotoconductive material along a longitudinal axis of thephotoconductive material. The light source is configured to illuminatethe photoconductive material from a first end of the photoconductivematerial so that the light from the light source propagatessubstantially parallel to a longitudinal axis of the photoconductivematerial. The forming the coaxial structure is performed using lasercutting or etching. The cylindrically symmetric outer conductor and thecylindrically symmetric inner conductor are formed using one or more ofe-beam deposition, sputtering, chemical vapor deposition,electrodeposition, or other method. The cylindrically symmetric innerconductor and the cylindrically symmetric outer conductor are formedfrom a conductive material such as: titanium, gold, aluminum, silver,platinum, chromium, or copper, or one or more conductive ceramicsincluding ITO, AZO, or FTO. The photoconductive material includes asemiconductor such as silicon, germanium, silicon carbide, diamond,gallium nitride, gallium arsenide, gallium phosphide, aluminum nitride,boron nitride, zinc oxide, gallium oxide, or cadmium telluride. A heightof the coaxial structure is determined based on an absorption of theincident light as is passes through the photoconductive material. Forexample, the height may be chosen so that 90%, or another percentage, ofthe incident light is absorbed as the incident light passes through thephotoconductive material. The cylindrically symmetric inner conductorand the photoconductive material are shaped to have a same predeterminedheight. The photoconductive switch module is configured to allow lightfrom a source to travel through the photoconductive material along alongitudinal axis of the photoconductive material. A light source isconfigured to illuminate the photoconductive material from a first endof the photoconductive material so that the light from the light sourcepropagates substantially parallel to a longitudinal axis of thephotoconductive material. The height value for the coaxial structure isselected to produce a predetermined absorption efficiency in thephotoconductive material. A thickness of the photoconductive material isequal to the difference between the outer radius of the inner conductorand the inner radius of the outer conductor, wherein the thickness isselected to produce a predetermined breakdown voltage. The height isselected to cause, at a predetermined minimum light intensity at theparticular wavelength or the range of wavelengths, the photoconductivematerial to break down. The outer electrode and the inner electrode mayhave a circular cross-sectional shape, a square cross-sectional shape,or any polygonal cross-sectional shape.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example of a photoconductive switch, in accordancewith example embodiments.

FIG. 2 depicts a cross-sectional view of an example of a photoconductiveswitch, in accordance with example embodiments.

FIG. 3 depicts a side view of an example of a photoconductive switch, inaccordance with example embodiments.

FIG. 4A depicts an example of a simulation showing the electricpotential, in accordance with example embodiments.

FIG. 4B depicts an example of a simulation showing the electric fieldstrength, in accordance with example embodiments.

FIG. 5 depicts a process, in accordance with some example embodiments.

FIG. 6 depicts a process for forming a photoconductive switch module, inaccordance with some example embodiments.

DETAILED DESCRIPTION

High voltage and high current switches have broad applicability inscience and industry. The ability to switch high voltages and highcurrents can be an enabling technology for pulsed systems such as radarsystems, communication systems, arbitrary-waveform generated high powerRF sources, amplifier systems, and many other applications. Disclosedherein are devices capable of switching high voltages and high currentsand methods of fabricating those devices.

In some example embodiments, a photoconductive switch includes twoconcentric cylinders that are conductive; an inner cylinder and an outercylinder. A photoconductive material fills a gap formed between theinner cylinder and the outer cylinder. As used herein, a cylinderincludes a tube and a hollow cylinder. The cylinder has a cross-sectionthat is circular or oblong. In some example embodiments, the interior ofthe inner cylinder is metal and in other example embodiments, the innercylinder is hollow. A bias voltage can be applied between the inner andouter cylinders. Light is incident on the photoconductive material. Whena sufficient amount of light per unit length is absorbed in thephotoconductive material, the photoconductive material becomesconducting thereby causing an electrical connection between the innercylinder and the outer cylinder. The photoconductive material may becomeconductive in a linear mode or an avalanche mode which is sometimesreferred to as breakdown. The absorption of the incident light in thephotoconductive material is determined at least in part by the physicalproperties of the photoconductive material and the wavelength of theincident light. Several factors influence the degree to which thephotoconductive material becomes conductive. The factors include thephysical properties of the photoconductive material, the length ofphotoconductive material, the bias voltage, the geometry of theelectrical contacts, and the wavelength of the incident light. The biasvoltage may affect the amount of light per unit length that must beabsorbed to cause conduction of the photoconductive material. In anotheraspect, a breakdown voltage is a voltage between the inner and outerconductors that when exceeded, the photoconductive material breaks downand conducts between the inner and outer cylinders. The breakdownvoltage is influenced by the physical properties of photoconductivematerial, the thickness of the photoconductive material between theinner and outer cylinders, the curvature of the inner cylinder, and thewavelength (or range of wavelengths) of the light on the photoconductivematerial. A smaller radius of curvature may have a lower breakdownvoltage than a larger radius of curvature because the electric fielddensity is higher for a smaller radius of curvature. A smaller thicknessof the photoconductive material may have a lower breakdown voltagebecause the field strength is inversely proportional to the thickness. Ahigher light intensity incident on the photoconductive material maycause a lower breakdown voltage.

In some example embodiments, an absorption efficiency of thephotoconductive material and a breakdown voltage can be chosenseparately. The absorption efficiency of the photoconductive switch canbe tailored or selected by tailoring or selecting the length of thephotoconductive material. The longer the length of the photoconductivematerial, the more of the incident light is absorbed (assuming theincident light source has suitable spectral content and intensity).Also, the longer the length of the photoconductive material, the higherthe current carrying capability between the inner and outer cylinders.Moreover, the breakdown voltage of the photoconductive material can beadjusted or selected based on the radii of the inner and outer cylindersand the difference in radii between the outer and inner cylinders. For afixed bias voltage, the smaller the radii of the inner cylinder, thehigher the electric field, and, thus, the photoconductive material iscloser to breakdown. Also, the thinner the photoconductive material (thedifference between the outer cylinder and inner cylinder radii), thehigher the field in the photoconductive material, and, thus, thephotoconductive material is closer to breakdown. In this way, theabsorption efficiency and the breakdown voltage of the photoconductiveswitch may be chosen separately. In some previous photoconductiveswitches, the absorption efficiency and breakdown voltage could not bechosen separately.

In some example embodiments, the photoconductive switch is referred toas being “axially pumped” because the coaxial photoconductive switch isilluminated along a longitudinal axis of the switch (corresponding tothe height of the cylinders). In previous switches, inefficientconversion of photons to electrons occurred due to the low absorptioncoefficient and short optical path length. Accordingly, a very highlaser fluence was needed, which caused degradation of the metal contactsof the switch. Fiber optics delivering the light to the switch couldalso be damaged. To fully absorb 90% of a 1 micron wavelength light,approximately 10 cm of material may be needed. However, to standoff 50kilovolts, only 200 microns of material may be needed (although thickermaterial may be used due to field enhancements at the contacts) whichabsorbs less than 1% of the incident light. By utilizing above bandgaplight (<375 nm) the absorption in 4H SiC may be 200 cm⁻¹ which maycorrespond to a 50 um depth. In this patent document, athree-dimensional coaxial geometry separates conflicting length scalesby applying the field in a direction orthogonal to the direction inwhich the light is absorbed. Additionally, the coaxial geometry confinesthe electric field within the rounded edges of the device which canreduce the field enhancements at the contacts thereby raising thebreakdown voltage of the device to closer to the material breakdownstrength. Operating at the breakdown strength of the material allows thespacing between the electrodes to be reduced, which in turn can reducethe laser energy needed by the reduction in electrode thickness spacingsquared.

FIG. 1 depicts a photoconductive switch system that can be used tofacilitate the understanding of the description that follows. Thephotoconductive switch includes an outer cylinder 110, an inner cylinder120, and a photoconductive material 150. A direct current (DC) biasvoltage 140 may be applied between the inner cylinder and outercylinder. Incident light 130 impinges at one end of photoconductivematerial 120. In some example embodiments, the photoconductive materialmay be illuminated from both ends. In some example embodiments, thephotoconductive material may be illuminated from one end and a mirrormay be placed at the other end of the photoconductive material causinglight reaching the mirror to be reflected back into the photoconductivematerial thereby improving the absorption in the photoconductivematerial of the incident light.

Inner cylinder 120 and outer cylinder 110 may be each have thicknesses.For example, outer cylinder 110 may have an inner radius and an outerradius. The difference between the outer radius of outer cylinder 110and the inner radius of the outer cylinder 110 is the thickness of outercylinder 110. In a similar fashion, the difference between the outerradius of inner cylinder 120 and the inner radius of the inner cylinder120 is the thickness of inner cylinder 120. The thicknesses of the outercylinder and the inner cylinder may be the same or may be different. Thematerial of the outer cylinder and the inner cylinder may be the same ormay be different. The inner and outer cylinders may include one or morematerials including titanium, gold, aluminum, platinum, chromium,copper, or other metal or any combination of metals. The material(s) mayalso include conductive ceramics such as indium tin oxide (ITO),aluminum doped zinc oxide (AZO), fluorine doped tine oxide (FTO), orother conductive material, or combination of materials.

Photoconductive material 150 may be placed between outer cylinder andinner cylinder 120. Photoconductive material 150 may have a thickness upto the difference between the inner radius of the outer cylinder 110 andthe outer radius of the inner cylinder 120. Photoconductive material 150may be a semiconductor material. Photoconductive and semiconductormaterials may include one or more of group IV materials such as silicon,germanium, silicon carbide, diamond; group III-V materials such asgallium nitride, gallium arsenide, gallium phosphide, aluminum nitride,boron nitride; II-VI materials such as zinc oxide, cadmium telluride; orany combination of the foregoing materials.

A bias voltage may be applied between outer cylinder 110 and innercylinder 120. For example, a voltage source 140 may be connected betweenthe outer cylinder 110 and the inner cylinder 120. The positive side ofthe voltage source 140, the anode 120A, may be connected to the innercylinder 120, and the negative side of voltage source 140, the cathode110A, may be connected to the outer cylinder 110. The bias voltage maybe connected with the anode connected to outer cylinder 110 and thecathode connected to inner cylinder 120 (not shown in FIG. 1).

Incident light 130 may illuminate photoconductive material 150. Incidentlight 150 may be absorbed by photoconductive material 150. The amount ofabsorption per unit length of photoconductive material 150 depends onthe physical propertied of photoconductive material 150 and thewavelength(s) of the incident light. The absorption rate may beindependent of bias voltage. The percentage of light absorbed per unitlength may be constant with intensity. For a given photoconductivematerial, various combinations of bias voltage 140 and intensity ofincident light 130 may cause conduction across photoconductive material150.

FIG. 2 depicts a cross-sectional view of an example of a coaxialphotoconductive switch, in accordance with some example embodiments. Thedescription of FIG. 2 also refers to FIG. 1. FIG. 2 includes an outercylinder 110 and an inner cylinder 120 with photoconductive material 150between the outer and inner cylinders. The inner cylinder has an outerradius, a, at 220, and the outer cylinder has an inner radius, b, at210. The photoconductive switch 200 is shaped such that inner cylinder120, outer cylinder 110, and photoconductive material 150 have a heighth (not shown in FIG. 2) perpendicular to the page of FIG. 2.Photoconductive material 150 fills the volume bound by the outer radius,a, of inner cylinder 120, inner radius, b, of outer cylinder 110, andheight h.

In operation, a voltage is applied between the inner cylinder 120 andouter cylinder 110 as described in FIG. 1. In this way, inner cylinder120, outer cylinder 110, and photoconductive material 150 may form acapacitor. The charge, Q of the capacitor is then related to the appliedvoltage with a relationship that may be expressed as Equation 1.

$\begin{matrix}{V = {{\int{Edr}} = {\frac{Q}{2\;\pi\; ɛ_{0}h}{\ln\left( \frac{b}{a} \right)}}}} & {{Eq}.\mspace{14mu}(1)}\end{matrix}$where, E is the electric field, h is the height of the electrode, and lnis the natural logarithm. The integral in Equation 1 is calculated overa radius, r, at 230 from a at 220 to b at 210. The electric field, E,may be expressed as Equation 2.

$\begin{matrix}{E = {\frac{Q}{2\;\pi\; ɛ_{0}r\; h} = \frac{V}{{\ln\left( \frac{b}{a} \right)}r}}} & {{Eq}.\mspace{14mu}(2)}\end{matrix}$

A smaller distance (b−a) between electrodes provides more current for agiven light intensity. A smaller distance (b−a) between electrodes maysupport a lower switch conduction voltage. The conduction voltage isalso dependent on the ratio of b/a. A larger ratio of b/a has a largerdifference in curvature between the inner and outer cylinders whichcreates a larger field enhancement at the inner cylinder. For example, aswitch with outer radius, b, of 1 cm and inner radius, a, of 0.5 cm andwith bias voltage 140 equal to 10,000 Volts experiences a maximum fieldof 28,853.9 Volts per centimeter.

In some example embodiments, the fraction of incident light that passedthrough the photoconductive material may be expressed as,

$\begin{matrix}{\frac{I}{I_{0}} = e^{- {ah}}} & {{Eq}.\mspace{14mu}(3)}\end{matrix}$where I is the intensity at a distance h through the photoconductivematerial from the incident light with initial intensity I₀, and a is theabsorption coefficient of the photoconductive material. In some exampleembodiments, h may be chosen so that I/I₀=0.10, or equivalently 90% ofthe incident intensity his absorbed in the photoconductive material inpath length h.

FIG. 3 depicts a side view of a photoconductive switch, in accordancewith some example embodiments. FIG. 3 depicts an outer cylinder 110, aninner cylinder 120, and photoconductive material 150 between the innerand outer cylinders. FIG. 3 depicts height, h, at 310. Cylinders 110,120, and photoconductive material 150 are each shown in FIG. 2 to be ofheight, h, 310. The height of photoconductive material 150 may be thesame or different from one or more of outer cylinder 110 and innercylinder 120.

FIG. 4A depicts a plot of electric potential and FIG. 4B depicts a plotof electric field strength for a coaxial example photoconductive switch,in accordance with some example embodiments. The description of FIGS.4A-4B also refer to FIGS. 1-3. FIGS. 4A-4B depict outer cylinder 120,inner cylinder 110, and the electric field strength between thecylinders. A shaded scale is shown at 410 indicating the field strength.The highest field strengths are closest to the inner cylinder 110 andthe lowest field strengths are closest to the outer cylinder 120. Movingfrom outer cylinder 120 to inner cylinder 110, the field strengthincreases. In some example embodiments, breakdown starts at a locationwhere the highest field strengths are present. In some exampleembodiments, edge effects may occur that cause field enhancementsthereby increasing the field strength at the edges. For example, edgeeffects may occur at the top and bottom of the inner and outercylinders. Higher field strengths due to edge effects may causeconduction at bias voltages lower than the breakdown voltage or withless incident light intensity.

Some example embodiments may be fabricated using a laser cuttingprocess. In one example fabrication process, the above-describedphotoconductive material is first in the shape of a block or rectangularprism. The block is then laser cut into a shape such as a cylinder. Aninner section of the cylinder is removed via laser cutting to produce anannulus shape. The interior and exterior of the annulus is then polishedand electrodes are deposited on the outer cylindrical surface and theinner cylindrical surface. The top and bottom surfaces of the annulusmay not have electrodes deposited but may have other layers applied suchas an anti-reflective coating. The electrodes may be deposited usinge-beam deposition, sputtering, chemical vapor deposition (CVD),electrodeposition, or other method. Bond wires or another contactmechanism may be used to connect a bias supply to the inner and outercylindrical surfaces.

Some example embodiments may be fabricated using etching. In oneexample, an etch mask is applied to a block of photoconductive material.The etch mask may be deposited or patterned onto the photoconductivematerial. The etch mask may be nickel or other mask material. Etchingetches away the photoconductive material except the annulus. Dry etchingor wet etching may be used. For example, inductively coupled plasma(ICP) etching, electron cyclotron resonance (ECR), or reactive-ionetching (RIE) may be used. The interior and exterior of the annulus isthen polished and electrodes are deposited on the outer cylindricalsurface and the inner cylindrical surface. The top and bottom surfacesof the annulus may not have electrodes deposited but may have otherlayers applied such as an anti-reflective coating. The electrodes may bedeposited using e-beam deposition, sputtering, chemical vapor deposition(CVD), electrodeposition, or other method. Bond wires or another contactmechanism may be used to connect a bias supply to the inner and outercylindrical surfaces.

FIG. 5 depicts a process, in accordance with example embodiments. At510, a first voltage referenced to a ground is be applied to an innercylinder. At 520, a second voltage referenced to the ground is beapplied to an outer cylinder. At 530, light illuminates aphotoconductive material filling a volume between the inner and outercylinders. The description of FIG. 5 also refers to FIGS. 1-4B.

At 510, a first voltage is applied to an inner cylinder. For example,inner cylinder 120 may be connected via a bond wire or other connectionto a voltage source referenced to a ground. For example, inner cylinder120 may be biased to a voltage of 5000 volts referenced to a ground.

At 520, a second voltage is be applied to an outer cylinder. Forexample, outer cylinder 110 may be connected via a bond wire or otherconnection to a voltage source referenced to the ground. For example,outer cylinder 110 may be biased to a voltage of 100 volts referenced tothe ground. In this example, the bias voltage applied across thephotoconductive material is 5000-100=4900 volts. Other voltages may beapplied to the inner and outer conductors to produce other biasvoltages. In some example embodiments, a negative bias voltage (outercylinder biased to a higher voltage than the inner cylinder) is appliedinstead of a positive voltage such as the foregoing example.

At 530, incident light is applied to a photoconductive material 150between the inner 120 and outer 110 cylinders. When the light has apredetermined wavelength or band of wavelengths, the photoconductivematerial 150 may conduct. Other wavelengths or bands of wavelengths maycause the photoconductive material to conduct at a higher or lower lightintensity. The bias voltage may affect the intensity needed to causeconduct. In some example embodiments, a higher bias voltage correspondsto avalanche conduction at a lower (or higher) light intensity.

FIG. 6 depicts a process for forming a photoconductive switch module, inaccordance with some example embodiments. FIG. 6 also refers to FIGS.1-5. At 510, the method includes forming a coaxial structure. At 520,the method includes selecting a height for the coaxial structure.

At 510, a coaxial structure is formed including a cylindricallysymmetric outer conductor, a cylindrically symmetric inner conductor,and a photoconductive material filling a volume between thecylindrically symmetric inner conductor and the cylindrically symmetricouter conductor. The cylindrically symmetric inner conductor, thecylindrically symmetric outer conductor, and the photoconductivematerial are configured as a photoconductive switch between thecylindrically symmetric inner conductor, the cylindrically symmetricouter conductor. In some example embodiments, the coaxial structureincludes outer cylinder 110, inner cylinder 120, and a photoconductivematerial 150 as described above.

At 520, a height value for the coaxial structure is selected. In someexample embodiments, the height is selected to produce an absorptionefficiency in the photoconductive material. In some example embodiments,other factors that may affect the height value include a thickness thatthe photoconductor may be grown, or a depth of a hole in the center ofthe photoconductive material.

In this patent document, the word “exemplary” is used to mean serving asan example, instance, or illustration. Any embodiment or designdescribed herein as “exemplary” is not necessarily to be construed aspreferred or advantageous over other embodiments or designs. Rather, useof the word exemplary is intended to present concepts in a concretemanner.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document.

What is claimed is:
 1. A high-voltage coaxial photoconductive device forswitching a voltage or a current, comprising: a cylindrical outerconductor having an inner radius; a cylindrical inner conductor havingan outer radius; a photoconductive material positioned between the outerradius of the inner conductor and the inner radius of the outerconductor, wherein the inner conductor and the outer conductor areconfigured to establish an electric field across the photoconductivematerial upon application of a voltage value between the inner and theouter conductors; and a light source having a particular wavelength orrange of wavelengths, the light source coupled to the photoconductivematerial to inject light into the photoconductive material, wherein thephotoconductive material is shaped to have a height to allow the lightto propagate from the light source through a substantial entirety of theheight of the photoconductive material and to be absorbed by thephotoconductive material with a corresponding absorption efficiency,thereby causing the photoconductive material to break down and to becomeconductive and a current to flow through the photoconductive materialbetween the inner conductor and the outer conductor, wherein thephotoconductive material is configured to break down at a breakdownvoltage based on properties of the photoconductive material and athickness of the photoconductive material, the thickness of thephotoconductive material determined based on the inner radius of theouter conductor and the outer radius of the inner conductor.
 2. Thecoaxial photoconductive device as in claim 1, wherein the innerconductor and the outer conductor are formed from a material thatincludes one or more of the following metals: titanium, gold, aluminum,silver, platinum, chromium, or copper, or one or more conductiveceramics including indium tin oxide (ITO), aluminum doped zinc oxide(AZO), or fluorine doped tine oxide (FTO).
 3. The coaxialphotoconductive device as in claim 2, wherein the inner cylinder isformed as a solid cylinder of the material.
 4. The coaxialphotoconductive device as in claim 1, wherein the photoconductivematerial includes one or more of: silicon, germanium, silicon carbide,diamond, gallium nitride, gallium arsenide, gallium phosphide, aluminumnitride, boron nitride, zinc oxide, or cadmium telluride.
 5. The coaxialphotoconductive device as in claim 1, wherein the height of thephotoconductive material is determined based on one or more of: thewavelength or range of wavelengths of the light source, the inner radiusof the outer conductor, the outer radius of the inner conductor, or thevoltage value between the inner and the outer conductors.
 6. The coaxialphotoconductive device as in claim 1, wherein the height for the coaxialstructure is selected to produce a predetermined absorption efficiencyin the photoconductive material.
 7. The coaxial photoconductive deviceas in claim 1, wherein the thickness of the photoconductive material isequal to the difference between the outer radius of the inner conductorand the inner radius of the outer conductor, and wherein the thicknessis selected to produce a predetermined breakdown voltage.
 8. The coaxialphotoconductive device as in claim 1, wherein the height is selected tocause, at a predetermined minimum light intensity at the particularwavelength or the range of wavelengths, the photoconductive material tobreak down.
 9. The coaxial photoconductive device as in claim 1, whereinthe outer conductor, the inner conductor, and the photoconductivematerial are each shaped to have the same height.
 10. The coaxialphotoconductive device as in claim 1, configured to allow light from thelight source travel through the photoconductive material along alongitudinal axis of the photoconductive material.
 11. The coaxialphotoconductive device as in claim 1, wherein the light source isconfigured to illuminate the photoconductive material from a first endof the photoconductive material so that the light from the light sourcepropagates substantially parallel to a longitudinal axis of thephotoconductive material.
 12. A high-voltage photoconductive device forswitching a voltage or a current, comprising: an outer electrode; aninner electrode; and a photoconductive material positioned between innerelectrode and the outer electrode, wherein: the inner electrode and theouter electrode are configured to allow an electric field to beestablished across the photoconductive material upon application of avoltage between the inner and the outer electrodes, and thephotoconductive material is shaped to have a height to allow, uponinjection of light having a particular wavelength or range ofwavelengths into the photoconductive material, the light to propagatethrough a substantial entirety of the height of the photoconductivematerial and to be absorbed by the photoconductive material with acorresponding absorption efficiency, thereby causing the photoconductivematerial to break down and a current to flow through the photoconductivematerial between the inner electrode and the outer electrode, whereinthe photoconductive material is configured to break down at a breakdownvoltage based on properties of the photoconductive material and athickness of the photoconductive material, the thickness of thephotoconductive material determined based on sizes of the innerelectrode and the outer electrode.
 13. The photoconductive device ofclaim 12, wherein the outer electrode and the inner electrode have acircular cross-sectional shape.
 14. The photoconductive device of claim12, wherein the outer electrode and the inner electrode have a squarecross-sectional shape.
 15. The photoconductive device of claim 12,wherein the outer electrode and the inner electrode have a polygonalcross-sectional shape.